Catalyst components of the Ziegler-Natta type for the stereospecific polymerization of olefins are widely known in the art. The latest developed catalysts for propylene polymerization comprise a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound and often with an external donor which is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility endowed with an intermediate molecular weight distribution.
Use of some phthalates however has been recently addressed as involving potential toxicity problems and therefore research activity have been devoted to find alternative classes of donor capable of replacing phthalates in terms of both performances and quality of the product.
One of the most interesting classes is that described in U.S. Pat. No. 7,388,061 disclosing esters belonging to the formula R1—CO—O—CR3R4-A-CR5R6—O—CO—R2 in which R1 and R2 groups, which may be identical or different, can be substituted or unsubstituted hydrocarbyl having 1 to 20 carbon atoms, R3-R6 groups, which may be identical or different, can be selected from the group consisting of hydrogen, halogen or substituted or unsubstituted hydrocarbyl having 1 to 20 carbon atoms, R1-R6 groups optionally contain one or more hetero-atoms replacing carbon, hydrogen atom or the both, said hetero-atom is selected from the group consisting of nitrogen, oxygen, sulfur, silicon, phosphorus and halogen atom, two or more of R3-R6 groups can be linked to form saturated or unsaturated monocyclic or polycyclic ring; A is a single bond or bivalent linking group with chain length between two free radicals being 1-10 atoms, wherein said bivalent linking group is selected from the group consisting of aliphatic, alicyclic and aromatic bivalent radicals, and can carry C1-C20 linear or branched substituents; one or more of carbon atoms and/or hydrogen atoms on above-mentioned bivalent linking group and substituents can be replaced by a hetero-atom selected from the group consisting of nitrogen, oxygen, sulfur, silicon, phosphorus, and halogen atom, and two or more said substituents on the linking group as well as above-mentioned R3-R6 groups can be linked to form saturated or unsaturated monocyclic or polycyclic ring.
The very broad formula reported encompass several structures with different performances. Within this general class the variety of performances is very high as some structures generate catalyst components showing activities and sterospecificities of interest while other structures make poor catalysts.
WO2011/068775 describes three and four atoms bridged dicarbonates compounds as internal donors in propylene polymerization. Some of the structures derive from the same diols disclosed in U.S. Pat. No. 7,388,061 with the difference that they are then formally esterified with chloroformate derivatives instead of aromatic acids (benzoic) derivatives. According to this patent the preferred structures are those in which the diol portion is part of an aromatic mono (phenyl) or polycyclic (naphthyl) group. The present applicant tests carried out on pentadiol dicarbonate derivative has shown very poor results indicating that for the same diol based structure, replacing the aromatic acid derivatives with the chloroformate derivatives carries a pronounced worsening of the properties particularly in terms of stereospecificity.
Based on this fact it has been very surprising to discover that mixed ester/carbonates of diols not only do not show worsening of the properties in respect of the analogue diesters but in some cases show an improvement of the same properties.